The present invention relates to three-dimensional waveguide structures, and more particularly to waveguides embedded in printed wiring boards that are able to operate at frequencies above 20 Ghz and to a method of manufacture thereof.
Devices for guiding the movement of electromagnetic waves are known as waveguides. Waveguides are generally enclosed spaces, which may be defined by encompassing walls formed of a material having a dielectric constant difference from that of the spaces. The cross-sectional dimensions of the waveguides depend on the frequency of the waves being guided and the desired transmission mode. Waveguides with improper dimensions may distort the signal passing therethrough. Where the frequency of the waves is very high, for example, above 20 Ghz, the cross-sectional dimensions of the waveguide must be very small, on the order of millimeters in size and may preclude the use of normal manufacturing methods.
Waveguides may be included in multi-layer printed wiring boards. As illustrated in FIG. 1, a multi-layered printed wiring board 10 may consist of plural dielectric layers 12, each sandwiched between two electrically conductive layers 14. Each of the electrically conductive layers 14 contains part of the circuit for the printed wiring board 10. In its most common use, electrical circuit paths are etched or formed onto the upper and/or lower surfaces of the board and electrical components are attached to the paths and interconnected thereby. Depending upon the type of board the intermediate conductive layers 14 may not be present. Propagation of electrical signals from one conductive layer 14 to another is often made by vias (not shown) which pierce through all or a portion of the board 10. A typical printed wiring board 10 for microwave and higher frequency signals may have three or four dielectric layers 12 and have a total thickness of 1 to 2 millimeters.
While various methods are known for making small waveguide cavities in printed wiring boards, none are known to provide cavity walls of the smoothness required to avoid the introduction of distorted or spurious signals. For example, it is known to build the waveguide cavity walls by layering successive depositions of conductive strips within the various dielectric layers. See, U.S. Pat. No. 4,647,882 to Landis. It is also known to form the waveguide cavity in a substrate material and to electroplate the walls of the cavity before the layers of substrate are placed together. See, for example, U.S. Pat. No. 3,157,847 to Williams. As is apparent, these processes may introduce non-uniformities into the wall of the waveguide cavity if each of the steps is not precisely carried out and the various layers are not stacked precisely upon each other. As such precision may substantially increase the cost of manufacturing such wave guides, it is desirable to develop a method of manufacturing a waveguide capable of operating at frequencies above 20 Ghz in which the manufacturing method does not require the precision of the prior art and does not produce the non-uniformities in the walls of the waveguide cavity.
A difficulty that often is presented when dealing with high frequency signals is the tendency of such signals to generate RFI/EMI (Radio Frequency Interference/Electromagnetic Interference) into nearby electrical components and signal paths and to be adversely affected by RFI/EMI from other sources. To prevent or reduce the RFI/EMI effect, it is known to construct special metal shields around electronic components and to attempt to segregate portions of circuits dealing with higher frequency signal from those portions of the circuit dealing with lower frequency signals. The former adds costs, uses valuables board space and may tend to overheat enclosed components; the latter is not always possible for all circuit designs and usually complicates the layout of the circuit on the wiring board.
Accordingly, it is an object of the present invention to provide a novel waveguide and method of manufacture thereof that obviates the problems of the prior art.
It is a further object of the present invention to provide a novel method of embedding a waveguide cavity and active and passive signal processing components in a printed wiring board.
It is still a further object of the present invention to provide a novel method of making a waveguide inside a printed wiring board in which the walls of the waveguide cavity are plated with an electrically conductive material after the layers of the printed wiring board have been joined together.
It is yet a further object of the present invention to provide a novel printed wiring board having a waveguide therein that has substantially seamless electroplated walls.
It is another object of the present invention to provide a novel method of isolating higher frequency signals from portions of an electrical circuit operating at a lower frequency.
It is yet another object of the present invention is to provide a novel printed wiring board having a waveguide cavity connected to frequency conversion components in the same dielectric layer as the waveguide.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.